ON THE EFFECT OF Ti 2 AlC ON THE FORMATION OF THERMALLY STABLE Mg NANO GRAINS Babak Anasori and Michel W. Barsoum Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA Keywords: Nanocrystalline Mg matrix composite, Liquid melt infiltration, Powder metallurgy. Abstract When Mg or Mg-alloys are reinforced with Ti 2 AlC – using a simple pressureless melt infiltration method – the result is nanocrystalline, nc, Mg-matrix composites, with outstanding mechanical properties. As an added bonus, the nc Mg-matrix is extraordinarily thermally stable. When AZ61 is used to infiltrate the Ti 2 AlC preforms, ultimate tensile stresses of 800 MPa are achieved. The reasons that lead to the formation of the nc-Mg are as of yet not understood. In this study, the different composites’ microstructures are investigated by X-ray diffraction, scanning and transmission electron microscopy. A correlation was found between the presence of oxygen and the nc Mg grains. Nanometer sized Mg grains were also found in between some of the Ti 2 AlC layers. Introduction Mg alloys have attracted a great deal of interest over the past decades due to their high specific strength, which renders them good candidates for applications in the aerospace and automotive industries [1, 2]. One method that can be used to achieve better properties is to fabricate materials wherein the grain size is at the nanometer scale [3]. The challenge for this approach, however, is that the grains tend to coarsen, even at room temperature [4]. Another approach to enhance the mechanical properties is to fabricate composites [5-7]. Herein, we show that by combining both approaches exceptional properties can be achieved. In 2009, we reported on the fabrication of novel, near net-shape nanocrystalline-Mg, nc-Mg, composites, reinforced with Ti 2 AlC [8]. The latter is a member of a large family of ternary carbides and nitrides, referred to as the MAX phases, with unique combinations of metal and ceramic properties. Like metals, they are electrically and thermally conductive, most readily machinable [9, 10], not susceptible to thermal shock, plastic at high temperatures, and exceptionally damage tolerant. Like ceramics, some of them are elastically rigid (Young’s mod. > 300 GPa), lightweight (≈ 4 g/cm 3 ) and maintain their strengths to high temperatures [11]. The ternaries Ti 3 SiC 2 and Ti 2 AlC are creep, fatigue and oxidation resistant [12,13]. The Ti-based MAX phases are also more conductive, both electrically and thermally, than Ti metal at all temperatures [14, 15]. Because of their layered structure, the MAX phases’ major deformation mechanism is basal slip, which in turn results in the formation of kink bands. Due to this deformation mechanism, they have excellent low and high-cycle fatigue and damping properties [16]. The latter increase with the applied stress squared [17]. The unusual damping properties have been explained by the formation and annihilation of incipient kink bands, IKBs. IKBs are parallel, co-axial, basal dislocation loops. More details on IKB-based deformation mechanisms can be found elsewhere [17, 18]. The near net shape nc-Mg composites were made by pressureless infiltration of Mg into a 50 vol.% porous Ti 2 AlC preform in a vacuum furnace at 750°C for 1 h. The fabrication details can be found elsewhere [8, 18, 19]. The resulting composites, with no additional steps, possessed outstanding properties. The best results were reported for a Mg-50 vol.% Ti 2 AlC, henceforth referred to as 50Mg-211. According to X-ray diffraction, XRD, transmission electron microscopy, TEM, and differential scanning calorimetery, DSC, results, the Mg grain size was found to be ≈ 35±15 nm. The density of the composite was 2.9 g/cm 3 . The ultimate compressive strength, UCS, was 700 ± 10 MPa [8, 20]. Although both starting components had tensile strengths lower than 300 MPa, the composite’s ultimate tensile strength, UTS, was 345 ± 40 MPa. This enhancement in UTS was ascribed to the small grain size of the Mg. In contrast to most nc-Mg microstructures, the nano grains were reported to be so exceptionally thermally stable that heating three times to 700°C – i.e. even melting the nc-Mg – did not lead to grain growth [20]. When the same infiltration method was used with Ti 3 SiC 2 as the reinforcement, the UCSs were not as high. The reason for this state of affairs was again related to Mg grain size, which was not at the nano scale in the Mg/Ti 3 SiC 2 composites [8]. More recently, we reported on 80 vol.% nc-Mg composites, reinforced with 20 vol.% Ti 2 AlC. In that case, the Mg average grain size was 90±15 nm. The density was 2 g/cm 3 [18]. The UCS was 350±10 MPa. Although this composite was comprised of 80 vol.% Mg, the nano grains were still thermally stable up to 700°C. The aim of this study is to explore: i) the reasons for the formation of the nc-Mg, and, ii) what keeps the grains from coarsening. XRD, scanning electron microscopy, SEM, and TEM results are discussed below. Experimental Details Both a 50Mg-211 and a Mg-50 vol.% Ti 3 SiC 2 composite, henceforth referred to as 50Mg-312, were fabricated. The processing method is detailed elsewhere [8, 18, 19]. XRD was carried out on a diffractometer (Model 500D, Siemens, Karlsruhe, Germany) and the patterns were collected using step scans of 0.01° in the range of 10°–80° 2θ and a step time of 2 s. Scans were made with Cu Kα radiation (40 kV and 30 mA). The polished surfaces were observed in a SEM (Zeiss Supra 50VP, Germany) and elemental maps were taken using the energy- dispersive spectroscope (EDS) (Oxford Inca X-Sight, Oxfordshire, UK) in the SEM.   TEM samples were prepared first by cutting ~300 μm thick slices from the bulk samples with a low speed diamond saw. Small 3 mm diameter disks were then made using an ultrasonic disk cutter (Model 170, Fischione, Export, PA). Both sides were polished with a dimpling grinder (Model 200, Fischione, Export, PA) using 3 μm, 1 μm and 0.1 μm diamond pastes successively. The disk Magnesium Technology 2012 Edited by: Suveen N. Mathaudhu, Wim H. Sillekens, Neale R. Neelameggham, and Norbert Hort TMS (The Minerals, Metals & Materials Society), 2012 409